Hi,
You should not frequently switch different brand formula on your baby. If you want to switch, you'd better graduately do it, not suddently change it all. It is especially for the very tiny one.
If your baby has be able to take some solid food, you can give s/he some pear or (Li(3) Zi(3)). Also let s/he do more activities. s/he will get some popo. It is based on my experience on my baby.